<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>使用 watch 实现计数器</title>
<script src="https://cdn.staticfile.org/vue/2.4.2/vue.min.js"></script>
</head>
<body>
<div id = "app">
 <p style = "font-size:25px;">计数器: {{ counter }}</p>
 <button @click = "counter++" style = "font-size:25px;">点我</button>
</div>
<script type = "text/javascript">
 var vm = new Vue({
    el: '#app',
    data: {
       counter: 1
    }
 });
 vm.$watch('counter', function(nval, oval) {
    alert('计数器值的变化 :' + oval + ' 变为 ' + nval + '!');
 });
</script>
</body>
</html>